Many industrial manufacturing processes generate wastewater containing metals and other contaminants; both organic and non-organic. Due to their inherent toxicity, regulatory authorities place strict limits on the maximum concentration of certain metals that can be legally discharged into the environment. In order to comply with these regulations, factories employ wastewater treatment processes to remove regulated substances from the wastewater. The two principal wastewater treatment methods are chemical precipitation and ion exchange.
Chemical precipitation is the most commonly used method today to remove dissolved (ionic) metals from wastewater. Chemical precipitation typically requires process operations of neutralization, precipitation, coagulation, flocculation, sedimentation, settling/filtration, and dewatering. It uses a series of tanks in which coagulants, precipitants and other chemicals such as polymers, ferrous sulfate, sodium hydroxide, lime, and poly aluminum chloride are added to convert metals into an insoluble form. In conjunction with adjusting the pH of the wastewater, this causes the metals to precipitate out of the water. Using a clarifying tank, the precipitates are allowed to settle, and then are collected as sludge; filtration can also be used to remove the solids. Excess water in the sludge is removed using filter presses and/or dryers. The sludge, which itself is a regulated hazardous waste, is then sent offsite where it is stabilized by mixing with cement or polymers, and then buried in a hazardous material landfill. In this fashion the concentrations of the regulated metals in the wastewater are reduced to a level in compliance with regulatory limits, allowing the water to be discharged from the facility. However, the need to handle, transport, and dispose of the resulting hazardous sludges is one of the most costly, labor intensive, resource demanding and difficult problems with chemical precipitation as a wastewater treatment.
The inherent disadvantage of chemical precipitation is that it is an active and additive process and, as such, requires that chemicals be added to the wastewater in order to remove regulated metals. The side effect of this is an increase in the concentrations of many other substances, as well as a deterioration in characteristics such as chemical oxygen demand (COD) and conductivity; thus requiring additional treatments and rendering the water unsuitable or uneconomical for recycling and reuse. Furthermore, the metals removed are not only unrecoverable, they are rendered into a regulated hazardous material requiring specialized disposal. As an additive process, chemical precipitation also increases, by orders of magnitude, the mass of waste material which needs to be handled, transported and landfilled.
As an active process, the effectiveness of chemical precipitation is predicated on the proper operational procedures and dosing of chemicals relative to fluctuating variables such as the number of metals in solution and their concentrations, as well as the presence and concentration of other substances. Underdosing of chemicals results in incomplete precipitation and removal of regulated metals, while overdosing wastes chemicals, generates additional volumes of sludge, and increases cost. Currently, due to the consequences of illegal discharges, most wastewater treatment operations simply absorb the additional cost and overdose the chemicals in their treatment operations. Also, as each metal optimally precipitates at a different pH, in wastewaters containing several metals, adjusting pH to precipitate one metal may actually cause another metal to resolubilize into the wastewater. Lastly, chemical precipitation processes require a large amount of floor space and capital equipment.
In contrast, ion exchange is a stoichiometrical, reversible, electrostatic chemical reaction in which an ion in solution is exchanged for a similarly charged ion in a complex. These complexes are typically chemically bound to a solid, insoluble, organic polymer substrate creating a resin; the most common of which is crosslinked polystyrene. Also, inorganic substrates like silica gel in various porosities and chemical modifications can be employed. Polystyrene crosslinking is achieved by adding divinyl benzene to the styrene which increases stability, but does slightly reduce exchange capacity. With a macro porous structure, these ion exchange resins are normally produced in the form of small (1 mm) beads, thus providing a very high and accessible surface area for the binding of the functional group complexes; the site where the ion exchange reaction actually occurs. The exchange capacity of the resin is defined by the total number of exchange sites, or more specifically, of its total available functional groups.
In the actual ion exchange reaction, an ion such as sodium (Na+) loosely attached to a functional group of the complex is exchanged for an ion in solution such as copper (Cu2+); that is, the sodium ions detach from the complex and go into solution while the copper ion comes out of solution and takes the place of the sodium ions on the complex. There are two types of ion exchange resins, cation exchangers, which exchange their positively charged ions (H+, Na+ etc.) for similarly charged ions (Cu2+, Ni2+, etc.) in solution, and anion exchangers, which exchange their negatively charged ions (OH−) for similarly charged ions in solution (chlorides, sulfates, etc.)
Ion exchange resins can also be selective or nonselective, based on the configuration and chemical structure of their functional groups. Non selective resins exhibit very similar affinities for all similarly charged ions, and consequently will attract and exchange all species without significant preference. Selective resins have specialized functional groups which exhibit different affinities to different ions of similar charge, causing them to attract and exchange ions with species in a well defined order of preference. The ion that is originally attached to the resin (e.g., H+, Na+, OH−) is of the lowest affinity, which is why it will exchange places with any other ion the resin encounters. Generally speaking, the relative affinity a resin exhibits for a particular ion is directly correlated to the exchange efficiency and capacity for that ion. However, as selective resins are based on relative affinities, the actual selectivity is also relative and not absolute.
Ion exchange resins can be regenerated once their capacity to exchange ions has been exhausted; that is, all of the functional groups have already exchanged their original ion for one which was in the solution. This is also known as a resin which has been “saturated” in that it cannot adsorb any additional ions. The process of regeneration is simply the reverse reaction of the original ion exchange. Clean water is first flushed through the saturated resin to remove any particles, solids, or other contaminations. A solution containing a high concentration of the original ion (e.g., the H+ ions contained in an acid) is then passed through the resin, causing the ion captured on the functional group (e.g., Cu2+) to forcibly detach from the functional group and solubilize into the solution and be replaced by the H+ ions from the acid. Depending on the type of resin (cation or anion, weak or strong) different chemicals are used to regenerate resins. In the case of selective or chelating resins, the strong affinities exhibited by these resins require greatly increased chemical consumption for the regeneration process. Regeneration results in a return of the resin to its original form (suitable for reuse) and a solution, also known as the regenerant, containing all of the metals or other ions stripped from the resin. Depending on its composition and complexity, some regenerants can be further processed by methods such as electrowinning to recover metals. The chemical consumption for regeneration as well as the difficulty and costs of treating or disposing of regenerants containing metals is the principal reason why ion exchange is often not a cost effective wastewater treatment option for metal bearing wastes.